With laser range finders, the distance between a measuring position and a target position is determined by measuring the time which the light of a laser pulse of a known rate of propagation requires in order to cover the distance between these two positions. A condition precedent is that the target is equipped with a retro-reflector which reflects back the light of the laser pulse in the direction of incidence and that the measuring position is configured so that the emitted light bundle can be separated optically from the received bundle.
For this purpose, it is known to use two separate transmitting and receiving telescopes or beam splitters. With large target distances such as when making distance measurements to satellites, it is a disadvantage that the large target distance imposes high requirements on the directional precision of the measuring position and the use of separate transmitting and receiving telescopes causes difficulty because of variations of their optical axes. Furthermore, the high energy losses associated with large target distances make it practically impossible to utilize conventional methods of physical beam splitting (partially transmitting mirrors) for separating transmitting and receiving beams.
Accordingly, in the measurement of large target distances, a state of the art has developed wherein a single, usually large telescope for transmitting and receiving and a geometrical separation method for the two bundles are utilized. The separation method provides that a rotating mirror having one aperture or a plurality of apertures is positioned along a diagonal in the beam path of the laser and that its rotational movement is so controlled that a laser shot of the laser beam just passes one of the apertures and the returning receiving beam impinges upon the reflecting surface of the rotating mirror. From there, the beam is guided in the direction of the receiver.
A disadvantage of this method is that the rotational speed of the rotating mirror must be very precisely synchronized with the laser frequency because the mirror will be destroyed if the laser beam itself with its high energy density impinges on the mirror. An expansion of the laser beam to reduce the energy density cannot help to reduce this disadvantage because the diameter of the aperture must then be correspondingly increased. This in turn requires an increase of the linear velocity because a full mirror surface must be made available to the reflecting beam and this makes synchronization difficult. A further disadvantage of this method is that the even rotation of the mirror can be realized only with a great effort so that no vibrations are transmitted to the aperture.